In analytical chemistry, liquid and gas chromatography techniques have become important tools in the identification of chemical sample components. The basic principle underlying all chromatographic techniques is the separation of a sample chemical mixture into individual components by transporting the mixture in moving fluid through a retentive media. The moving fluid is called the mobile phase and the retentive media is the stationary phase. One of the differences between liquid and gas chromatography is that the mobile phase is either a liquid or gas, respectively.
In a gas chromatograph ("GC"), a supply of inert carrier gas(mobile phase) is continually passed as a stream through a heated column containing porous media coated with the sorptive stationary phase. Alternatively, a GC column comprises a hollow capillary tube having an inner diameter of from 50 to several hundred microns. A sample of the subject mixture is injected into the mobile phase stream and passed through the column. As the subject mixture passes through the column, components of the sample adhere at differential rates to the wall of the capillary tube or the sorptive media. Separation is due primarily to differences in the adhesion characteristics of the sample's active components with respect to the column. A detector, positioned at the outlet end of the column, detects each of the separated components as it exits the column.
The analytical choice between liquid and gas chromatography techniques is largely dependent on the molecular weight of the components being analyzed. Liquid chromatography can analyze much heavier compounds than gas chromatography. However, gas chromatography detection techniques are more sensitive and therefore generally preferred.
The accuracy of any analysis performed by a GC is dependent upon an accurate knowledge of the amount of carrier gas and sample gas flowing into and through the GC. Systems having valves and flow sensors for measuring the flow rate of the carrier gas, the flow rate of the sample, and the pressure of both are known. One GC having such valves and sensors is described in U.S. Pat. No. 4,948,389.
Although the flow and pressure sensors can be calibrated, long term drift can degrade the accuracy of both these sensors over time. All attempts to reduce the sensitivity of the flow and pressure sensors to the effects of long term drift have been unsuccessful. The problem is almost certainly related to the basic process and materials used to fabricate the sensors. The problem of sensor degradation over time has not been solved in the known art.